Add versioning to Ledger tables on-disk storage

[jasagredo]

In order to identify incompatible changes in the on-disk storage of LedgerTables, we decided to add versioning. On an old version detected, we will choose to either delete the snapshot or to upgrade the snapshot to the newer version if it is easy enough.

We also need golden tests to identify such changes, but they have to be exhaustive in the possible stored values, so they will need to be crafted with care.